Three patients were transported to area hospitals (2 adults and 1 infant) for evaluation. The infant was transported as a precautionary measure only. Occupants of 2912 were complaining of headache and nausea when they called 911. There were no CO detectors in the residence at the time of the call. The quick action of the residents prevented a potentially tragic outcome. The Fire Marshal's investigation revealed the source of the CO leak to be a malfunctioning hot water heater. Washington gas responded to the scene and evaluated the hot water heater. Residents were allowed to reoccupy the town home when CO levels were back to normal.